Knob configured for torque control

A reference frame assembly for an arthroplasty, the reference frame assembly comprising:

• an optical tracker;
• a rod coupled to the optical tracker and extending therefrom;
• a base configured for coupling to an anatomy of a patient;
• and
• a knob for selectively coupling the rod with the base to mount the optical tracker in a desired position relative to the anatomy of the patient, the knob comprising:
• an outer housing including a portion configured as a handle;
• a first magnet positioned within the outer housing;
• a second magnet positioned within the outer housing, wherein the second magnet and the first magnet are configured to exert a repulsive force with respect to one another;
• a first crown positioned within the outer housing and restrained from translation relative to the outer housing, wherein the first crown includes a first plurality of teeth on a first side thereof;
• and
• a second crown coupled to the second magnet and positioned within the outer housing, wherein the second crown is rotatable with the outer housing, wherein the second crown has a second plurality of teeth on a second side thereof that interface with and selectively engage the first plurality of teeth on the first side of the first crown;
• wherein the second crown overcomes the repulsive force between the first magnet and the second magnet and translates relative to the outer housing and the first crown when the first crown is threaded onto the base to a predetermined torque.

A torque limiting system, comprising:

• a knob configured to threadably engage a first medical instrument and couple with a second medical instrument, the knob comprising:
• an outer housing including a portion configured as a handle;
• a first magnet positioned within the outer housing;
• a second magnet positioned within the outer housing, wherein the second magnet and the first magnet are configured to exert a repulsive force with respect to one another;
• a first crown positioned within the outer housing and restrained from translation relative to the outer housing;
• and
• a second crown coupled to the second magnet and positioned within the outer housing, wherein the second crown is rotatable with the outer housing and is configured to selectively engage the first crown to apply a torque thereto;
• wherein the knob is configured to maintain a co-rotating engagement of the second crown with the first crown using the repulsive force between the first magnet and the second magnet until the torque reaches a predetermined torque between threads of the first crown and corresponding threads of first medical instrument whereby the second crown overcomes the repulsive force to translate relative to the first crown and the torque applied to the first crown by the second crown substantially ceases.